A National Historic Landmark District (NHLD) is a historic district that has received similar recognition. The district may include contributing properties that are buildings, structures, sites or objects, and it may include non-contributing properties. Contributing properties may or may not also be separately listed.

In 1960 the National Park Service took on the administration of the survey data gathered under this legislation, and the National Historic Landmark program began to take more formal shape.[4] When the National Register of Historic Places was established in 1966, the National Historic Landmark program was encompassed within it, and rules and procedures for inclusion and designation were formalized. Because listings (either on the National Register, or as an NHL) often triggered local preservation laws, legislation in 1980 amended the listing procedures to require owner agreement to the designations.[5]

On October 9, 1960, 92 properties were announced as designated NHLs by Secretary of the Interior Fred Andrew Seaton. The first of these was a political nomination: the Sergeant Floyd Grave and Monument in Sioux City, Iowa was officially designated on June 30 of that year, but for various reasons, the public announcement of the first several NHLs was delayed.

About half of the National Historic Landmarks are privately owned.[8] The National Historic Landmarks Program relies on suggestions for new designations from the National Park Service, which also assists in maintaining the landmarks. A friends' group of owners and managers, the National Historic Landmark Stewards Association, works to preserve, protect and promote National Historic Landmarks.

^The counts and locations of NHLs are described most accurately in List of National Historic Landmarks by state. This extends, and corrects errors from, the National Park Service's "National Historic Landmarks Survey List of National Historic Landmarks by State", also referenced.